In a scanning electron microscope (“SEM”), a primary beam of electrons is scanned upon a region of a sample that is to be investigated. The energy released in the impact of the electrons with the sample causes the emission of x-rays and secondary electrons. The quantity and energy of these x-rays and secondary electrons provide information on the nature, structure and composition of the sample. The term “sample” is traditionally used to indicate any work piece being processed or observed in a charged particle beam system and the term as used herein includes any work piece and is not limited to a sample that is being used as a representative of a larger population. The term “secondary electrons” as used herein includes backscattered primary electrons, as well as low energy electrons originating from the sample. To detect secondary electrons, a SEM is often provided with one or more electron detectors.
One type of electron microscope in which the sample is maintained in a gaseous environment is a High Pressure Scanning Electron Microscope (HPSEM) or Environmental Scanning Electron Microscope. Such a system is described, for example, in U.S. Pat. No. 4,785,182 to Mancuso et al., entitled “Secondary Electron Detector for Use in a Gaseous Atmosphere.” An example is the Quanta 600 ESEM® high pressure SEM from FEI Company.
In an HPSEM, the sample is maintained in a gaseous atmosphere having a pressure typically between 0.01 Torr (0.013 mbar) and 50 Torr (65 mbar), and more typically between 1 Torr (1.3 mbar) and 10 Torr (13 mbar). The region of high gas pressure is limited to the sample region by one or more pressure-limiting apertures that maintain a high vacuum in the focusing column. By contrast, in a conventional SEM the sample is located in vacuum of substantially lower pressure, typically less than 10−5 Torr (1.3×10−5 mbar).
In an HPSEM, secondary electrons are typically detected using a process known as “gas ionization cascade amplification” or “gas cascade amplification,” in which the secondary charged particles are accelerated by an electric field and collide with gas molecules in an imaging gas to create additional charged particles, which in turn collide with other gas molecules to produce still additional charged particles. This cascade continues until a greatly increased number of charged particles are detected as an electrical current at a detector electrode. In some embodiments, each secondary electron from the sample surface generates, for example, more than 20, more than 100, or more than 1,000 additional electrons, depending upon the gas pressure and the electrode configuration. In some embodiments positive gas ions or photons generated in the gas cascade are detected instead of electrons and used to generate an image. The term “gas cascade amplification imaging” as used herein refers to images generated using any combination of these three imaging signals. The term “gas cascade detector” as used herein refers to a detector that can be used to detect any combination of these three imaging signals.
One advantage of an HPSEM as compared to a conventional SEM is that the HPSEM offers the possibility of forming electron-optical images of moist samples, such as biological samples, and other samples which, under the high vacuum conditions in a conventional SEM, would be difficult to image. An HPSEM provides the possibility of maintaining the sample in its natural state; the sample is not subjected to the disadvantageous requirements of drying, freezing or vacuum coating, which are normally necessary in studies using conventional SEMs. Another advantage of an HPSEM is that the ionized imaging gas facilitates neutralization of electrical charges that tend to build up on insulating samples, such as plastics, ceramics or glasses.
While an HPSEM can observe moist biological samples, problems still exist with such observations. For example, when hydrated materials are observed at room or body temperature, water tends to condense on all surfaces within the sample chamber. Such condensation can interfere with the operation of HPSEM, as well as cause corrosion and contamination.
Charged particle beams, such as electron beams or ion beams, can also be used to induce a chemical reaction to etch a sample or to deposit material onto a sample. Such processes are described, for example, in U.S. Pat. No. 6,753,538 to Mucil et al. for “Electron Beam Processing.” The process of a charged particle beam interacting with a process gas in the presence of a substrate to produce a chemical reaction is referred to as “beam chemistry.” The term “processing” as used herein includes both processing that alters the sample surface, such as etching and deposition, as well as imaging. The term “processing gas” is used to include a gas that is used for imaging or a gas that is used together with the charged particle beam to alter the sample. The term “imaging gas” is used to include a gas that is used for gas cascade amplification imaging. The classes of gasses are not mutually exclusive, and some gases may be used for both altering the sample and for forming an image. For example, water vapor can be used to etch a sample that includes carbon and can be used to form an image of samples that include other materials.
Generally, the applicability of existing HPSEM & VPSEM tools for gas imaging is limited, in part, by several factors: First, low secondary electron (SE) image quality relative to that of high vacuum SEM. This problem is a consequence of the fact that the gas cascade is a poor amplifier of the SE imaging signal (compared to amplifiers employed by high vacuum SE detectors). Second, a lack of imaging gases that can be used for high quality imaging of liquids and dynamic processes in non-aqueous environments. Third, problems caused by residual contaminants present in vacuum systems that frequently use low-volatility (“sticky”) imaging gases such as H2O and NH3.